1. Field of the Invention
The present invention relates to a tone generator for an electronic musical instrument, and more particularly to a delay-feedback-type tone generator.
2. Prior Art
The tone generator, which is conventionally used for the electronic musical instrument, employs a mixing method utilizing a frequency modulation. According to this mixing method, a frequency modulation is performed on signals, which represent sine waves and are read from memories, so as to create overtone components. The sounds which are obtained by performing the above-mentioned mixing method may be heard as if they are monotonous, or mathematically mixed. In short, it is difficult to create the sounds, whose properties are similar to those of the acoustic sounds, by using the above mixing method. Thus, the recent technology has developed a new musical tone synthesizing apparatus which instead of using the modulation when mixing the waveforms, activates a physical model simulating a tone-generation mechanism of an acoustic musical instrument. This kind of apparatus has been disclosed in Japanese Patent Publication No. 58-48109, for example. In this apparatus, there is provided a loop circuit which contains a delay circuit and a filter and to which a signal, such as an impulse signal, representing an initial waveform containing a plenty of frequency components is applied. Then, the signal circulating through the loop circuit is extracted as a musical tone signal. According to the above-mentioned apparatus, every time the initial-waveform signal applied to the loop circuit passes through the filter, the certain frequency characteristic is imparted to the initial-waveform signal by the filter. Due to such frequency characteristic, certain frequency components, included in the initial-waveform signal, are attenuated. As a result, it is possible to obtain an attenuating sound from the output of the loop circuit, wherein the attenuating sound is a sound whose level is attenuated in a lapse of time and whose tone color is altered in a lapse of time.
The tone generator conventionally known suffers from a problem that the overtone components become monotonous or are artificially regularized. In other words, this tone generator is not suitable for synthesizing the musical tones whose properties are similar to those of the acoustic sounds so that the tone color is altered in a complex manner.